Cobalt oxide decorated octahedral ceria hollow structures (CoOx/CeO2) with various contents of CoOx nanoparticles were prepared via a simple chemical impregnation method. The obtained samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), UV-Vis diffuse-reflectance spectroscopy (DRS), surface photovoltage spectroscopy (SPV) and transient photovoltage spectroscopy (TPV). The photocatalytic oxygen evolution via water oxidation was investigated for the as-prepared CoOx/CeO2 nanocage composites. The photocatalytic results indicate that the CoOx/CeO2 nanocage composite with 1 mol% CoOx shows the highest photocatalytic activity. The excellent photocatalytic activity can be attributed to the improved visible-light absorption of CoOx/CeO2 composites and the efficient separation of excited electron-hole pairs between CoOx and CeO2, which can effectively enhance the lifetime of charge carriers in the CoOx-modified samples and then improve the oxygen evolution activity. Cobalt oxide is expected to be an excellent water oxidation co-catalyst for semiconductor photocatalysts.